MVA (Multi-domain Vertical Alignment) type liquid crystal display devices have wider viewing angle characteristics than TN type liquid crystal display devices. MVA type liquid crystal display devices, therefore, are widely used as liquid crystal display devices for TVs and the like (see Patent Documents 1 and 2, for example). The entire contents disclosed in Patent Documents 1 and 2 are hereby incorporated herein by reference.
MVA type liquid crystal display devices have domain control structures (also called alignment control structures), which are disposed on a pair of substrates facing each other to sandwich a vertical alignment type liquid crystal layer, on the sides facing the liquid crystal layer. With the domain control structures, a plurality of liquid crystal domains having different director alignment directions (tilt directions) are formed. The domain control structures are slits (openings) provided in the electrode or dielectric protrusions (ribs) formed on the electrode on the side facing the liquid crystal layer.
Typically, on each of the pair of substrates, a domain control structure having linear segments extending in two directions crossing each other at a right angle is disposed. When a pair of the substrates are observed from an angle perpendicular to the substrates, the domain control structure formed on one substrate and the domain control structure formed on the other substrate are arranged alternately and in parallel with one another. As a result, when a voltage is applied to the liquid crystal layer at a given pixel, four domains, in which liquid crystal molecules fall in different directions (also called the liquid crystal domain director directions) that are about 90° apart, are formed between the linear domain control structure segments. Typically, four liquid crystal domains are formed, where respective liquid crystal domain directors form a 45° azimuth angle to the polarizing axis (transmission axis) of a pair of polarizing plates disposed in a crossed Nicols arrangement. When azimuth angle of 0° is the polarizing axis direction of one of the polarizing plates (horizontal direction of the display surface (3 o'clock direction of the clock's face), for example) and when the counterclockwise rotation is defined as the positive direction, the azimuth angles of the four liquid crystal domain directors are 45°, 135°, 225°, and 315°. The azimuth angle is hereinafter defined as stated above, unless otherwise indicated.
“Pixel” herein refers to the smallest display unit of the liquid crystal display device. In the case of color display devices, “pixel” refers to the smallest display unit of each primary color (typically, red, green or blue), and is also called “dot.”
In general, pixels are arranged in a matrix of rows and columns. Here, “row direction” refers to the horizontal direction of the display surface (azimuth angle is 0° or 180°), and “column direction” refers to the vertical direction of the display surface (azimuth angle is 90° or 270°). A pixel includes a pixel electrode, a liquid crystal layer, and an opposite electrode (common electrode) facing the pixel electrode sandwiching the liquid crystal layer. The pixel electrode has edges (sides) extending in the row direction, and edges (sides) extending in the column direction. The linear domain control structures included in an MVA type liquid crystal display device have segments extending in two directions that cross each other at a right angle to form the above-mentioned four liquid crystal domains. They extend, for example, in the azimuth directions of 45° (225°) and 135° (315°). That is, segments of the linear domain control structure (or structure extension) on the opposite electrode, which segments extending in two directions crossing each other at a right angle, intersect with pixel electrode edges extending in the row or the column directions.
When a potential difference is formed between the pixel electrode and the opposite electrode, a diagonal electrical field (fringe field) is formed near the edges of the pixel electrode. The diagonal electrical field formed along the edge of the pixel electrode makes liquid crystal molecules fall in a direction that crosses the electrode edge at the right angle. Therefore, in the area around the intersection of the domain control structure (or its extension) provided on the opposite electrode and the edges of the pixel electrode extending in the row or column direction, the diagonal electrical field formed near the edge of the pixel electrode disturbs the alignment of the liquid crystal molecules controlled by the domain control structure. If the alignment of the liquid crystal molecules is disturbed, naturally, the display quality is deteriorated.
In order to suppress the disturbance in the liquid crystal molecules alignment in the area around the intersection of the domain control structure (or its extension) disposed on the opposite electrode and the pixel electrode edge extending in the row or column direction, Patent Document 1 discloses a configuration in which a linear supplemental structure is disposed at a location facing the edge of the pixel electrode where the alignment disturbance occurs such that the structure extends in parallel with the edge of the pixel electrode. The supplemental structure may be disposed inside the pixel or outside the pixel. The supplemental structure may be, for example, slits formed in the opposite electrode or dielectric protrusions formed on the opposite electrode on the side facing the liquid crystal layer, whichever used as the domain control structure on the opposite electrode. That is, if the domain control structure is the slits formed on the opposite electrode, slits are used as the supplemental structure, and if the domain control structure is the dielectric protrusions formed on the opposite electrode on the side facing the liquid crystal layer, dielectric protrusions are used as the supplemental structure.
However, because the area where the supplemental structure (slits or dielectric protrusions) is formed does not contribute to the display, any portion of the supplemental structure present in the pixel reduces the transmission accordingly. Also, although disturbance in the alignment of the liquid crystal molecules near the pixel electrode edge is suppressed by the supplemental structure, reduced transmission is unavoidable, because the alignment direction of the liquid crystal molecules around the edge is different from the direction of the domain director set by the domain control structure.
Recently, in order to improve the viewing angle dependency of γ characteristics of the MVA type liquid crystal display device, the applicant disclosed in Patent Document 3 a liquid crystal display device and a driving method that can improve the viewing angle dependency of γ characteristics by dividing a single pixel into a plurality of sub-pixels having different brightness. In particular, the viewing angle dependency of γ characteristics that raises the display luminance of low gradation higher than the prescribed luminance (display becomes whitish) can be improved. Such display or drive may herein referred to as “area gradation display,” “area gradation drive,” “multi-pixel display,” “multi-pixel drive,” or the like. The entire contents disclosed in Patent Document 3 are hereby incorporated herein by reference.
Patent Document 3 discloses a liquid crystal display device in which single auxiliary capacitance is provided for a plurality of sub-pixels disposed in a pixel, making an auxiliary capacitance opposite electrode (which is connected to the CS bus line) constituting the auxiliary capacitance electrically independent on a sub-pixel by sub-pixel basis. In this liquid crystal display device, by changing the voltage supplied to the auxiliary capacitance opposite electrode (this is called auxiliary capacitance opposite voltage), different effective voltages are applied to the liquid crystal layer for the individual sub-pixels using the capacity division. Today, MVA type liquid crystal display devices are used to provide the multi-pixel displays for purposes requiring a wide viewing angle characteristics, such as TVs, and the multi-pixel displays are performed in a variety of manners.
Patent Document 4 discloses an MVA type liquid crystal display device having another multi-pixel structure. In the liquid crystal display device disclosed in Patent Document 4, the CS bus line is disposed in parallel with the source bus line extending in the column direction. Two sub-pixel electrodes are provided in each pixel (called first and second pixel electrodes in Patent Document 4), and the overlapping area of one of the sub-pixel electrodes and the CS bus line, and the overlapping area of the other sub-pixel electrode and the CS bus line are different. Because of the difference in the overlapping area of the CS bus line and the individual sub-pixel electrodes, the two sub-pixels have different auxiliary capacitances. As a result, a multi-pixel structure is realized. Approximately rectangular pixels are disposed such that the long sides extend in parallel with the row direction. A pixel disposed such that the long side extend in parallel with the row direction is herein called “horizontally long pixel,” and a pixel disposed such that the long side extend in parallel with the column direction is herein called “vertically long pixel.”
In a liquid crystal display device having the multi-pixel structure described above, a pixel electrode is divided into a plurality of sub-pixel electrodes that correspond to the plurality of sub-pixels. That is, the plurality of sub-pixel electrode constitute a pixel electrode. A plurality of sub-pixel electrodes are sometimes provided in a pixel for purposes other than formation of the multi-pixel structure. For example, in some cases, a plurality of sub-pixel electrodes constitute a pixel electrode to make it easy to repair any short circuit failure between the pixel electrode and the opposite electrode, or to prevent any short circuit failure from becoming highly visible. In this case, an equal voltage is supplied to the plurality of sub-pixel electrodes included in each pixel.